1. Field of the Invention
The present invention relates to an ignition device for bus connection, a plurality of the ignition devices being connected to an ignition control system via a common bus, and the ignition devices being selectively operable by means of electrical energy and an electrical signal supplied from the ignition control system.
2. Description of the Related Art
An airbag device mounted in an automobile is arranged so that an airbag is inflated by high pressure gas generated by combustion of a propellant, and an ignition device (an igniter or a squib) for igniting the propellant is provided in the airbag device. The ignition device is connected to an ignition control system, into which an acceleration signal generated upon a vehicle collision is input. Energizing an ignition element of the ignition device generates heat to ignite the propellant, thereby inflating the airbag.
Recent automobiles are equipped with a large number of airbag devices such as an airbag device that deploys from a steering wheel, an airbag device that deploys from a dashboard, an airbag device that deploys from a seat side part, and an airbag device that deploys from a roof side part. Therefore, the number of circuits within the ignition control system have to be increased in line with the number of the airbag devices, and each time the number of airbag devices increases the ignition control system has to be changed even for the same type of vehicle. In each case the production cost increases. Furthermore, if the ignition control system and each of the airbag devices are connected by means of a harness exclusively used therefor, the harness becomes very long, and it is difficult to ensure space for arranging the harness.
U.S. Pat. No. 5,760,489 discloses an arrangement in which a plurality of airbag devices are connected to a common bus extending from an ignition control system, electrical energy for ignition is supplied from the ignition control system to an ignition device of each of the airbag devices, and an electrical signal is also supplied for operating only the ignition device of a predetermined airbag device among the plurality of airbag devices.
In this case, the ignition device of each of the airbag devices requires a communication circuit for communicating with the ignition control system and an ignition circuit for making an ignition element generate heat when it receives an ignition command during the communication. Such an ignition device comprising a communication/ignition circuit is known from U.S. Pat. No. 6,418,853.
Furthermore, when a plurality of airbag devices are simultaneously operated, the sum of electrical energy supplied from the ignition control system to each of the ignition devices increases. In this regard, an ignition element that can generate heat with low electrical energy is known from U.S. Pat. No. 5,847,309.
In the arrangement disclosed in Japanese Patent Application Laid-open No. 2000-241098, the ignition device (an igniter 1) is provided separately with the communication/ignition circuit (an electronic card 17) for outputting an ignition signal in communication with the ignition control system (a central control unit 110), and the ignition element (a resistive heating element 11), which is operated by the ignition signal output from the communication/ignition circuit so as to ignite a propellant (a pyrotechnic ignition composition 7). Therefore, not only do the dimensions of the ignition device increase, but also it is necessary to connect a wire between the communication/ignition circuit and the ignition element when assembling the ignition device, thus disadvantageously increasing the number of assembly steps.